Acid fracturing is a widely used technique for increasing the production of oil from a well that penetrates an underground limestone or dolomite hydrocarbon bearing formation. Typically during an acid fracturing treatment, a pad fluid is rapidly injected into the formation so as to create a buildup in wellbore pressure sufficient to overcome compressive stresses and tensile strength of the rock formation. When subjected to a sufficient pressure, the rock fails allowing a crack, also referred to as a fracture, to form in the formation. Continued fluid injection often increases the fracture's length, height and width. Acid is then injected into the fracture and the acid chemically reacts with the face of the fracture. The reaction of the acid with face of the fracture etches the face so that when the fracture closes flow channels are created that extend deep into the formation. If the acid fracturing treatment is properly done, these flow channels remain open when the well is placed back on production, thus increasing the productive capacity of the well.
Increased production of oil, also known as stimulation of the well, is achieved by either creating a flow path through a damaged zone around the well bore or by altering the flow pattern in the reservoir. Typically small volume acid treatments can overcome well bore damage and restore native productivity to a well by removing flow restrictions caused by a zone of low permeability near the wellbore. However in order to alter the flow pattern in the reservoir, a much larger volume acid treatment process is typically required.
Practical and economic formation stimulation requires the proper selection of acid type as well as the acidizing technique utilized. Acid systems currently in use can be broadly classified into the following groups: mineral acids which includes hydrochloric acid, hydrofluoric acid and mixtures of hydrochloric-hydrofluoric acid; organic acids which include formic acid; and acetic acid; powdered solid acids such as sulfamic acid, and chloroacetic acid; mixed acid systems such as acetic-hydrochloric acid, formic-hydrochloric acid; formic-hydrofluoric acid; retarded acid systems for example gelled acids, chemically retarded acids, and emulsified acids. All of these different acid systems with the exception of hydrochloric acid-hydrofluoric acid and formic-hydrofluoric acid mixtures are used to stimulate carbonate formations.
The distance that reactive acid moves along a fracture during treatment is called the acid penetration distance. The acid penetration distance is one of the variables that will determine the success or failure of the treatment. The acid penetration distance may be effected by the fluid loss characteristics of the acid, the width of the fracture, the rate of acid reaction with the formation rock, the temperature of the acid and the surrounding formation, and the acid flow rate along the fracture. The effective acid penetration is not necessarily the penetration of the acid fluid that is being pumped. It is the penetration achieved before the fluid turns neutral (through reaction with the fracture wall) and loses it's ability to etch the entire remaining length of the fracture.
The fluid loss characteristics of any particular fluid depends upon the porosity of the formation, the viscosity of the fluid and the presence or absence of any fluid loss control agents in the fluid. Generally, fluid loss to the formation increases with increasing formation porosity, decreases with increasing fluid viscosity (i.e. the more viscous the lower the fluid loss) and decreases with the presence of fluid loss control agents such as barite, humates, polymer resins and the like. When acid enters a fracture it reacts with the fracture walls and in doing so eliminate the filtercake created by fluid loss additives used in the pad fluid. Once this occurs the fracture geometry will be controlled by the fluid loss characteristics of the acid. It has been reported in the literature that the use of an effective fluid loss additive in the acid is critical to maximizing the acid penetration distance. Fluid loss control of acid in carbonates is generally much more difficult to obtain than with sandstone formation because the acid continuously dissolves the rock matrix that supports the fluid-loss additive.
It is reported in the literature that generally, an increase in fracture width normally will increase the distance reactive acids will penetrate along the fracture, (i.e. the acid penetration distance. One example given shows that an increase in the fracture width from 0.05 to 0.20 inches can increase the acid penetration distance from 80 to 175 ft. in a limestone formation. Thus being able to keep the fracture open for the longest possible time is important to the success of the acid treatment. The difficulty in keeping a fracture open is that as the acid dissolves the formation in close proximity to the well, and thus increased volumes of fluid are needed to maintain the pressure necessary to keep the fracture open. Thus the interrelationship between the rate of fluid lose to the formation (fluid loss characteristics) and the ability to keep a fracture open. Generally the distance reactive acid will penetrate along a fracture will increase with an increase in the flow velocity of the acid along the fracture. An increase in injection rate also can also reduce the temperature at which the acid enters the fracture, thus increasing the acid penetration distance by reducing the reaction rate.
The temperature of the fluid in the fracture has a significant influence on the geometry and acid penetration distance. Generally it is not accurate to assume that the temperature of the fluid being injected is approximately that of the formation after it enters the formation. Because of the differences in heat capacity and fluid loss characteristics, the temperature within the fracture will depend on whether or not a pad fluid is used and if so the characteristics of the pad fluid. Generally fluids with a low viscosity and a high rate of fluid loss from the formation can effectively cool the formation surrounding the fracture. However, because of the high rate of fluid loss, the fracture is likely to be narrow and will require large volumes of fluid in order to increase fracture length. Fluids of this type are often used as precooling pad fluids to reduce the temperature of the fracture and thus increase acid penetrations distance.
Viscous fluids with low fluid loss to the formation typically do not cool the formation as much as a less viscous fluid. Under such circumstances the fluid in the fracture will be approximately that of the formation. In cases in which the acid reacts rapidly with the formation, the temperature has little effect on the acid penetration distance because the rate of reaction is controlled by the mass transport of the acid to the formation face. However, with slower reactions, the acid penetration distance will vary more with temperature.
It should be understood by one of skill in the art that acids tend to react faster with limestone formations than with dolomite formations. The primary reason for this is the difference in the chemical composition of the two different types of rock. At low formation temperatures it is reported in the literature that acid penetration distance will be greater for dolomite than for a limestone formation. In contrast, due to the higher the reaction rate the acid penetration distance in a limestone formation is independent of temperature.
The selection of the acid utilized during the acid treatment of the formation can have a significant impact on the outcome of the treatment. If the fluid loss rate of an acid can be controlled, it is sometimes possible to use a reduced reactivity (i.e. retarded) acid to maximize the acid penetration distance. Acids may be retarded for acid fracturing purposes only if their reaction rate during flow along the fracture is significantly lower than the reaction rate of hydrochloric acid alone.
There are several types of retarded acid systems known in the art including: viscous acid systems, gelled acid systems, chemically retarded acid systems, and organic acid systems. Viscous acids include emulsified acids and acids gelled with guar or other polymers. Typically the acid (28% hydrochloric acid) is mixed with kerosene or other suitable oil to form either an acid external phase emulsion or an acid internal phase emulsion. The retardation provided by emulsified acids is primarily a result of the high emulsion viscosity, which reduces the rate of mass transfer to the fracture wall. Shielding by the oil layer may also provide a measure of reduction in the reaction rate. When viscous acids are used often very large volumes of fluid are needed in order to assure adequate fracture conductivity. Gelled acids are commonly prepared by adding polymers such a guar, gum karaya or polyacrylamide to hydrochloric acid. The resulting viscous acid is retarded so long as the fluid is viscous. Unfortunately, the viscosity of the gelled acid is quickly lost because the gelling agent typically degrades due to elevated temperature encountered in the well bore and the formation. The highly temperature dependent nature of gelled acid often makes it use undesirable from a field operation and cost effectiveness viewpoint. Chemically retarded acids such as those containing oil wetting surfactants have been reported as being effective in reducing the reactivity of hydrochloric acid in laboratory tests. However, at typical field flow rates, the retarding effect of the oil wetting surfactants is not realized and the acid reacts as would regular hydrochloric acid.
Organic acids such as acetic acid and formic acid have been use either alone or in mixtures with hydrochloric acid. These mixtures of acids have been reported as being useful as retarded acid systems. Generally the reaction rate for organic acids is lower than for hydrochloric acid. However, the penetration distance for acetic acid or formic acid are similar to conventional hydrochloric acid.
In summary of the above, acid fracturing remains a technology in which there exists a continuing need for new and improved methods for delivery of the acid to the formation and controlling the reaction of the acid within the formation.